When a single-point brightness pattern is on presenting, an LED display scheme will appear a plurality of slightly bright points around the single-point, which is commonly known as a ghost. A reason of a ghost generation is, there is a plurality of Cp and Cn parasitic capacitances on a row and a column of a plurality of scanning lines on a PCB, when a system is switching a state, an incorrect voltage of the Cp and Cn may cause a leakage current generating, while a plurality of LEDs around the bright spot may further generate an abnormal light leakage.
Referencing to FIG. 1, the ghost may be divided into an upper ghost and a lower ghost, a mechanism of generating the ghost is as follows:
State 1: when a first row of the scanning lines COM1 finishes displaying, a parasitic capacitance Cp1 of the first row of the scanning lines is charged by a transistor of P1 and kept in a high voltage. When a next line COM2 (a second row of the scanning lines) starts, if a SEG2 lights up a Q22, then a plurality of charges of the parasitic capacitance Cp1 in the first row of the scanning lines will be discharged by the SEG2 through a Q12, thus generating a light leakage from the Q12, called an upper ghost.
State2: The Q22 is lit up, when the COM2 finishes displaying, a parasitic capacitance Cn2 of a second column of the scanning lines is discharged by a transistor of N2 and kept in a low voltage, and at a short time of turning on a COM3, a current charges and lifts the voltage of Cn2 from a P3 through a Q32, thus generating a light leakage from the Q32, called a lower ghost. Wherein from Q11 to Q33, all are light emitting diodes.
All above, the ghosts are all generated by a plurality of abnormal voltages from the parasitic capacitances in the system, thus, an appropriate bias voltage helps to eliminate a phenomenon of the ghost.
In the industry, improving the lower ghosts is currently achieved by pulling high a voltage of a SEG terminal via a column constant current drive chip before switching a COM line to keep the parasitic capacitances Cn of the columns of the scanning lines in a high voltage, keeping no current from charging the Cn when switching between the COM lines, therefore eliminating the lower ghosts.
According to a problem of the upper ghosts, when designing a circuit for an LED displayer, a process of discharging the rows of the scanning lines will be made, that is, a blanking circuit will be added. And in the present arts, a blanking circuit has a plurality of schemes as listed below:
1. pull down a ground resistance;
2. a Zener diode+a pull-down resistance.
Currently, on a market, all solutions of eliminating the upper ghost are achieved by discharging the rows of the scanning lines, which needs to add a plurality of additional circuits, causing a plurality of problems including a caterpillar effect and a reverse bias voltage of an LED lamp bead.
Specifically, the scheme 1 has a plurality of characters including a low cost and a convenience, however, to a plurality of rows of the scanning lines, it may have a constant pull-down current, generating a pretty large reverse bias voltage to the LED lamp beads and having a caterpillar phenomenon exist. The scheme 2 suppresses the rows of the scanning lines to 3.3V, without any caterpillar effects, however, it may still generate a pretty large reverse bias voltage to the LED lamp beads, and the scheme has a plurality of components.
Therefore, the current technology needs to be improved and developed.